Generally speaking, an airbag comprises three parts. The bag itself is made of a thin, nylon fabric, which is folded into the a vehicle part such as a steering wheel or dashboard. The sensor triggers the inflation of the bag, which happens upon a collision impact, such as running into a wall at 10 to 15 miles per hour. An inertia switch or a mechanical switch is tripped when there is a mass shift that closes an electrical contact, indicating that a crash has occurred. The sensors receive information from an accelerometer built into a microchip. A propellant is used to inflate the airbag. According to Wikipedia, the decision to deploy an airbag in a frontal crash is made within 15 to 30 milliseconds after the onset of the crash, and both the driver and passenger airbags are fully inflated within approximately 60-80 milliseconds after the first moment of vehicle contact. Naturally, if an airbag deploys too late or too slowly, the risk of occupant injury from contact with the inflating airbag may increase. The airbag sensor is a MEMS accelerometer, which is a small integrated circuit with integrated micro mechanical elements. The microscopic mechanical element moves in response to rapid deceleration, and this motion causes a change in capacitance, which is detected by the electronics on the chip that then sends a signal to fire the airbag. One common MEMS accelerometer in use is the ADXL-50 by Analog Devices, but there are other MEMS manufacturers as well.
According to Wikipedia, initial attempts using mercury switches did not work well. Before MEMS, the primary system used to deploy airbags was called a “rolamite”. A rolamite is a mechanical device, consisting of a roller suspended within a tensioned band. As a result of the particular geometry and material properties used, the roller is free to translate with little friction or hysteresis. This device was developed at Sandia National Laboratories. The rolamite, and similar macro-mechanical devices were used in airbags until the mid-1990s when they were universally replaced with MEMS.
From the onset of the crash, the entire deployment and inflation process is about 0.04 seconds. Because vehicles change speed so quickly in a crash, airbags must inflate rapidly to reduce the risk of the occupant hitting the vehicle's interior.
According to Wikipedia, when the frontal airbags are to deploy, a signal is sent to the inflator unit within the airbag control unit. An igniter starts a rapid chemical reaction generating primarily nitrogen gas (N2) to fill the airbag making it deploy through the module cover. Some airbag technologies use compressed nitrogen or argon gas with a pyrotechnic operated valve (“hybrid gas generator”), while other technologies use various energetic propellants. Propellants containing the highly toxic sodium azide (NaN3) were common in early inflator designs. However, propellants containing sodium azide were widely phased out during the 1990s in pursuit of more efficient, less expensive and less toxic alternatives. The azide-containing pyrotechnic gas generators contain a substantial amount of the propellant. The driver-side airbag would contain a canister containing about 50 grams of sodium azide. The passenger side container holds about 200 grams of sodium azide. The alternative propellants may incorporate, for example, a combination of nitroguanidine, phase-stabilized ammonium nitrate (NH4NO3) or other nonmetallic oxidizer, and a nitrogen-rich fuel different than azide (e.g. tetrazoles, triazoles, and their salts). The burn rate modifiers in the mixture may be an alkaline metal nitrate (NO3-) or nitrite (NO2-), dicyanamide or its salts, sodium borohydride (NaBH4), etc. The coolants and slag formers may be e.g. clay, silica, alumina, glass, etc.[31] Other alternatives are e.g. nitrocellulose based propellants (which have high gas yield but bad storage stability, and their oxygen balance requires secondary oxidation of the reaction products to avoid buildup of carbon monoxide), or high-oxygen nitrogen-free organic compounds with inorganic oxidizers (e.g., di or tricarboxylic acids with chlorates (ClO3-) or perchlorates (HClO4) and eventually metallic oxides; the nitrogen-free formulation avoids formation of toxic nitrogen oxides). See Liquid propellant airbag inflator with dual telescoping pistons U.S. Pat. No. 6,039,347, hereby incorporated by reference.
While a concerted effort has been made to make and install air bags in automobiles, little has been done as far as wearable, expandable protection devices. Helmet technology generally focuses on the inside of the helmet, where little space is available, as opposed to outside of the helmet protection where space is not a factor.
Clothing is intended to function as a covering for the purposes of preserving body temperature, without providing shock absorption protection. In the case of circus performers, attention is given to ground covering, yet none is given to wearable protection. Airbag suits have also been developed for use by Motorcycle Grand Prix riders, as disclosed in Motorcycle News Dainese airbag suit in action 21 Nov. 2007. They are connected to the motorcycle by a cable and deploy when the cable becomes detached from its mounting clip, inflating to protect the back.